was reporting an unexpected exit status: 139 instead of 110.
This was caused by a seg fault in distccd, deep in the bowels of
vsnprintf(), which appears to be due to calling vsnprintf() twice
on the same va_list.  The fix is to use va_copy() in src/trace.c.

Of course it's never quite as easy as that.  va_copy() exists
only in C99, not in C89.  Some implementations have __va_copy()
but not va_copy().  So we need to autoconf it.  There was
already an autoconf test for this, but only for __va_copy.
I've moved the code which defined VA_COPY from snprintf.c to
a new header file va_copy.h, and added a VA_COPY_END macro.

Also, fix another bug that I noticed at the same time:
snprintf.c was using va_copy(), but was not matching each
call to va_copy with a corresponding call to va_end(),
as required by the C99 standard.